Copper metabolism in small ruminants : with special emphasis on goats

Abstract

Twenty-four 1 – 1½year-old castrated Cashmere goats and 24 1½year-old Corriedale wethers were individually penned and offered ryegrass-white clover silage at maintenance level. Mineral content of basal silage was 4.01 mg Cu, 0.26 g S, 0.20 mg Mo, 40.1 mg Zn and 121 mg Fe/kg DM. Six animals of each species were randomized to constitute a treatment group which was either offered the basal diet or the basal diet enhanced with 5, 15 or 25 mg Mo plus 5 g S/kg DM during a copper depletion phase. Liver biopsies were obtained 54 and 19 days, respectively in goats and sheep prior to housing then subsequently on days 28, 56 and 84 during depletion. Plasma samples were obtained fortnightly for determination of total plasma Cu, TCA-soluble copper, direct reacting copper and caeruloplasmin and whole blood for superoxide dismutase. The animals were subsequently repleted using injectable calcium copper edetate (Coprin; 1 ml = 50 mg Cu). The parameters mentioned above, excepting SOD, were monitored together with liver specific enzymes including aspartate amino transferase (AST), gamma-glutamyl transferase (Symbol to be inserted here-GT), alanine amino transferase (ALT) and sorbitol dehydrogenase (SDH).

Supplementing Mo + S into silage induced increases of total plasma copper concentration which was TCA insoluble and decreased caeruloplasmin levels. Both the basal and Mo + S enhanced silage induced liver Cu depletion; on a percentage basis goats in control, low, medium and high depletion treatment groups respectively lost 80.04, 72.2, 77.7 and 79.6% of initial liver copper reserves. Respective percentage losses for sheep were 52.6, 49.0, 57.8 and 70.0%. Rates of reduction in liver Cu concentration were 0.48, 0.41, 0.43 and 0.82 mg Cu/kg DM/day for the control, low, medium and high treatment groups, respectively for goats, 1.22, 1.26, 1.54 and 1.94 mg Cu/kg DM/d, respectively in sheep. The respective rates of loss of liver Cu content in goats and sheep were 2.9, 2.4, 2.5, 4.6 and 3.8, 5.7, 4.7 and 5.9 µg/kg LW.day. Whereas the rates of liver Cu depletion seemed to relate to level of dietary supplement of Mo + S in sheep there were clear linear relationships between initial liver Cu concentration and rates of loss in both goats (r = .967) and sheep (r = .782). Strong linear relationships between initial liver Cu content and rates of loss were also high (r = .964 and r = .669) in goats and sheep, respectively. The only observable symptom of copper deficiency was oculofacial achromotrichia in four coloured goats.

Goats became Cu depleted earlier than sheep. They were then injected with Coprin at 1.0, 1.5 and 2.0 ml/goat which meant they received 1.32 - 8.04 mg Cu/kg LW. Of the 18 injected goats 14 died. The 4 surviving had received 1.32, 2.43, 3.35 and 3.72 mg Cu/kg LW. Enzyme activity of AST and [Symbol to be inserted here] -GT increased above normal in both those dying and those surviving but ALT was within normal ranges in survivors and only increased in those dying. SDH was not measured during the crisis.

For about a month sheep were subjected to intensive copper depletion both by drenching with ferric ammonium citrate at 500 and 1000 mg Fe/kg DM feed and injection of ammonium tetra-thiomolyhdate at 3.0 mg TTM/kg LW/d in split application. Fifteen copper depleted sheep were used in a comparative copper repletion with 5 Cu depleted and 9 replacement goats with low liver copper (7.6 - 27.5 mg/kg DM). Coprin dosages were adjusted per unit liveweight (mg Cu/kg LW). Two goats were allocated to control, six were injected at 0.5 mg and six at 1.0 mg Cu/kg LW. Of the 15 sheep (5 groups of 3) one remained as control and the others received Cu EDTA at rates of 0.5, 1.0, 2.0 and 3.0 mg Cu/kg LW of injectable Cu EDTA. Animals receiving 0.5 and 1.0 mg Cu/kg LW survived and the amounts of liver copper stored were dose-related in both goats (r = .999) and sheep (r = .994). Only one sheep survived the 2.0 mg Cu/kg LW dosage. Increases in the liver specific enzymes (AST, SOH and [Symbol to be inserted here] -GT) occurred above the normal ranges in sheep, when both species were injected with calcium copper edetate at 1.0 mg Cu/kg LW, whilst remaining within the normal ranges in goats. This suggested that goats were less susceptible to toxicity from injected copper than sheep at a similar dosage per unit liveweight.

Animals dosed at 0.5 or 1.0 mg Cu/kg LW declined in liver Cu content as day 7 and day 28 postinjection samples revealed: Rates of loss were 10.6 to 24.4 and 16.4 to 40.5 µg/kg LW.day for goats and sheep, respectively. The rates of decline in liver Cu content from day 7 to day 28 postinjection were linearly related to day 7 contents in both goats (r = .608) and sheep (r = .847).

When rates of liver Cu loss were calculated in relation to liveweight there was evidence for a higher rate of loss in goats, on the basis of a given liver Cu concentration, particularly at higher liver Cu concentrations during both the Cu depletion and repletion phases.